A term referring to someone addicted to psychoactive drugs, opiates in particular, and especially those using heroin (an opiate). The term was coined during WW1 when heroin or morphine addicts collected and sold scrap metal (junk) for a higher price to buy their drugs. After this, heroin and morphine themselves began to be known as "junk" and the users of said drugs became known as "junkies".

He's been slamming M for 2 months now...He's starting to seriously become a junky.